Mechanisms of metabolism other than those mediated by the mixed function oxidases may be important in activating certain chemicals to their ultimate carcinogenic form. Prostagladin H synthetase is being used to activate compounds in the mouse mammalian cell forward mutation assay. Initial experiments showed hydrogen peroxide to be a good substrate for this enzyme. However, mutagenicity results are confounded by the interaction of hydrogen peroxide with sodium pyruvate, a media component in this assay. Other substrates are also being examined. Minimal treatment conditions and optional concentrations of purified enzyme have been determined. The possible mechanisms responsible for the formation of mutagenic metabolites induced by various peroxidase enzyme systems are being investigated. Selective inhibitors of the peroxidase are used to aid in the elucidation of these mechanisms. Studies include the identification of metabolites induced by this activation system.